cambium

cambium (kămˈbēəm) [key], thin layer of generative tissue lying between the bark and the wood of a stem, most active in woody plants. The cambium produces new layers of phloem on the outside and of xylem (wood) on the inside, thus increasing the diameter of the stem. In herbaceous plants the cambium is almost inactive; in monocotyledonous plants it is usually absent. In regions where there are alternating seasons, each year's growth laid down by the cambium is discernible because of the contrast between the large wood elements produced in the spring and the smaller ones produced in the summer. These are the annual rings, by which the age of a tree can be established. A tree dies when it is "ringed," or girdled, i.e., cut through the cambium layer. The cork cambium, which lies outside the phloem layer, produces the cork cells of bark.